Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. In each technology, ink is fed through channels formed in a printhead. Each channel includes a nozzle from which drops of ink are selectively extruded and deposited upon a medium. When color printing is desired, each technology typically requires independent ink supplies and separate ink delivery systems for each ink color used during printing.
The first technology, commonly referred to as “drop-on-demand” ink jet printing, provides ink drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
Conventional “drop-on-demand” ink jet printers utilize a pressurization actuator to produce the ink jet drop at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink drop to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink drop to be expelled. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source which produces a continuous stream of ink drops. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink drops. The ink drops are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink drops are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink drops are not deflected and allowed to strike a print media. Alternatively, deflected ink drops may be allowed to strike the print media, while non-deflected ink drops are collected in the ink capturing mechanism.
U.S. Pat. No. 3,878,519, issued to Eaton, on Apr. 15, 1975, discloses a method and apparatus for synchronizing drop formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, issued to Hertz, on Aug. 24, 1982, discloses a method and apparatus for controlling the electric charge on drops formed by the breaking up of a pressurized liquid stream at a drop formation point located within the electric field having an electric potential gradient. Drop formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the drops at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect drops.
U.S. Pat. No. 4,638,382, issued to Drake et al., on Jan. 20, 1987, discloses a continuous ink jet printhead that utilizes constant thermal pulses to agitate ink streams admitted through a plurality of nozzles in order to break up the ink streams into drops at a fixed distance from the nozzles. At this point, the drops are individually charged by a charging electrode and then deflected using deflection plates positioned the drop path.
As conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes in which to operate. This results in continuous ink jet printheads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink drops from a filament of working fluid and deflect those ink drops. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink drops and non-printed ink drops. Printed ink drops flow along a printed ink drop path ultimately striking a print media, while non-printed ink drops flow along a non-printed ink drop path ultimately striking a catcher surface. Non-printed ink drops are recycled or disposed of through an ink removal channel formed in the catcher.
U.S. Pat. No. 6,497,510, issued to Delametter et al., on Dec. 24, 2002, discloses a geometry of printhead employing asymmetrically applied heat for continuous ink jet printer systems in which the improvement is an enhanced lateral flow in the ink channel near the entrance to the nozzle bore. This enhanced lateral flow within the printhead serves to lessen the amount of heat needed per degree of angle of deflection of drops which have been ejected from the printhead.
U.S. Pat. No. 6,450,619, issued to Anagnostopoulos et al., on Sep. 17, 2002, discloses a continuous ink jet printhead incorporating nozzle bores, heater elements, and associated electronics which may be made at lower cost by forming the heater elements and nozzle bores during the processing steps used to fabricate the associated electronics, for example, by CMOS processing. More expensive MEMS type processing steps are thereby kept to a minimum. Structures are provided to increase the lateral flow near the entrance to the nozzle bore.
U.S. Pat. Nos. 6,213,595 and 6,217,163, issued to Anagnostopoulos et al., on Apr. 10 and Apr. 17, 2001 respectively, disclose a continuous ink jet printhead incorporating a heater having a plurality of selectively independently actuated sections which are positioned along respectively different portions of the nozzle bore's perimeter. By selecting which segments are to be actuated (and optionally adjusting the power level to different segments), the drop placement may be more accurately controlled.
U.S. Pat. No. 6,505,921, issued to Chwalek et al., on Jan. 14, 2003, discloses an embodiment of a continuous ink jet printing system incorporating a heater near the nozzle bore, the volume of each ink drop broken from the ink stream being determined by the frequency of activation of the heater; and further incorporating a gas flow which deflects droplets of one size into a nonprinting path, while droplets of another size are allowed to strike the recording medium.
It may be appreciated that low cost, excellent image quality, high printing throughput, and high reliability are important advantages for a continuous ink jet printing system. Further improvements are desired in printhead fabrication simplicity and cost, especially those improvements which are compatible with the integration of driving and control electronics required for precise droplet control of a large number of nozzles at high resolution. In addition, to prevent image quality from degrading due to obstructions in the ink flow path in the printhead, it is desirable to provide a printhead geometry and a method for cleaning the printhead which facilitate removal of such obstructions.